1. Field of the Invention
The present invention relates to a method and to an apparatus for measuring and controlling blow-up of a foam sheet of a plastic material being heated on a thermoformer oven, and more particularly, controlling the oven operation through monitoring the material being heated in the thermoformer oven through sensing the blow-up thereof on the forming station in order to be able to regulate the thickness or blow-up of the plastic material prior to its conveyance into the molding or forming station.
In recent years, molded articles which are constituted of a thermoplastic foam material have gained wide commercial and consumer acceptance due to their economics in manufacture, aesthetic appeal and excellent physical properties. Among such articles are egg cartons, meat and food trays, cups and even larger items, such as boat hulls, refrigerator liners or the like, which are readily molded from a thermoformable plastic material. For instance, it is possible to mass-produce thermoformed articles in an extremely economical manner by heating a continuous sheet of a thermoplastic foam material, for example, foamable polystyrene or similar materials, to a predetermined temperature in a thermoformer oven so that the material is in a thermoplastic state, and from there to convey the heated sheet into a molding station or forming press of a thermoformer, in which the desired thermoformed article configurations are molded into the sheet of the plastic foam material.
In essence, the heating to the molding temperature of the thermoplastic foam material in the thermoformer oven not only plasticizes the material but concurrently therewith expands the foam material such that the thickness of the material may be increased, for instance, up to as much as approximately 1.5 to 2.0 times the original thickness during its conveyance through the oven. Although this phenomenon, known as "sheet blow-up" is essentially three-dimensional in nature, orientation stresses purposely built into the material during extrusion thereof, opposed by the clamping action of the sheet gripping and conveying mechanism in the thermoformer oven, and the continuity of the sheet in the machine direction, result in minimal changes in the sheet length and width during heating of the latter in the thermoformer oven. The increase in the sheet thickness during heating is desirable since it reduces the density of the foam material and allows for the subsequent filling of wider mold gaps at various portions of specific foam plastic articles.
The mold gap in the production tooling, in effect, the void in the mold which is occupied by the product, may vary at different locations throughout the total mold cross-section for various reasons relating to desired final product shape, strength, functional needs, or aesthetics. Thickness of the expanded foam prior to entry into the mold is important since it is desirable to fill all portions of the mold gap throughout the various cross-sections of the mold cavity, and this must be accomplished while counteracting the thinning of the heated material as various portions are stretched and formed into the desired contours of the finished article.
In order to be able to provide molded products in the molding station which evidence a high degree of definition in their conformance with the configurations of the mold, and which provide a consistency and attractive appearance essential to their saleability and utility by a consumer, it is necessary to heat the plastic material to a predetermined temperature in the thermoformer oven and to maintain the material at that temperature to thereby achieve the desired extent of sheet "blow-up" or expansion. Variations in the temperature over the surface of the heated sheet of plastic material as it is conducted from the thermoformer oven into the forming station or molding press of the thermoformer will adversely affect the uniformity in the foam material thickness and, as a consequence, in a like manner the consistency and quality of the thermoformed articles.
Generally, the thermoformable foam plastic sheet is heated by radiant energy heating during its conveyance through the thermoformer oven through the intermediary of a series of heater elements which are usually arranged above and below the path of travel of the material. Such heater elements, which direct the radiant heat against the surfaces of the sheet of plastic material, may be constituted of suitable parabolic heaters, bar heaters, infrared heaters, or ceramic blocks having heating coils therein, which normally extend transverse of the direction of travel of the plastic material sheet through the thermoformer oven, and with the heater elements being energized from an external power source. Additional heater elements may be provided along the edges of the oven to compensate for heat losses to gripping devices which engage the edges of the sheet for transporting the latter through the thermoformer oven. Upon leaving the thermoformer oven, the thus heated thermoplastic sheet is conveyed to a forming station in which it is introduced into a thermoforming press and interposed between the surfaces of a pair of mold members which cooperate to force or mold the heated thermoplastic material into the shape of the thermoformed articles as defined by the contours of the mold surfaces.
2. Discussion of the Prior Art
At this time, apparatus and methods have been developed or employed which will measure the blow-up or thickness expansion of a foam plastic material as it is heated in a thermoformer oven. Generally, temperature conditions within the thermoformer oven, which are assumed to be indicative of the temperatures to which the plastic material has been heated and the blow-up thereof, are usually monitored by temperature sensors or thermocouples which measure the temperature of one of the heater elements in each heat zone within the thermoformer oven; of particular importance being the temperature of the material at the discharge end of the thermoformer oven. Any differentials in the monitored or sensed temperatures which deviate from desired temperature levels are read by an operator who will then manually regulate the energy input to various of the heater elements in order to either raise or lower the temperature in those zones of the oven so as to provide and maintain the optionally desired thermoplastic material temperatures and resultant blow-up.
Thus, Holden U.S. Pat. No 4,438,054, assigned to the common assignee of this invention, measures the thickness of a thermoformable sheet material at a plurality of locations within a thermoformer oven, and thereafter regulates the oven temperature in response to the sensed thickness to maintain the requisite blow-up.
Other publications which are directed to monitoring and/or controlling the thickness or blow-up of a thermoformable material are DiBiasi et al. U.S. Pat. Nos. 4,412,961; Burry 4,376,085; Daubenbuchel, et al. 4,382,761 and Cisar, et al. 4,454,082. Moreover, such sensing may be carried out through the utilization of different types of thickness measuring devices, including mechanical feelers or sheet contacting shoes, or electrical and/or thermal sensors, such as LVDT (linear variable differential transformers) as described in Maples U.S. Pat. No. 4,505,158, or sensors such as thermocouples.
Although monitoring or sensing the temperature conditions within the thermoformer oven and controlling the amount of radiant heat generated by the heater elements will, in theory, be an indication of the plastic material temperatures and, as a result, the theoretical "blow-up" or increase in thickness of the material, in actuality considerable deviations have been encountered in the thickness of the foamed material after heating in comparison with the presumed final ideal thickness of the material. These differences between the actual blow-up thickness and the intended or contemplated thickness of the foam plastic material may be occasioned, among other factors, by the cycling on and off of the heaters during normal operation, low or high spot temperatures at various locations within the oven, or varying conditions within the oven, all of which tend to adversely affect the uniformity or extent of heating of the plastic material and, as a result, the extent of blow-up and consistency of any thermoformed articles produced from the non-uniformly heated material, possibly resulting in voids, uneven articles and poor molded definition of the product.
While the actual thickness of the expanded foam material is of particular importance relative to the quality and shape of the resultant thermoformed article, heretofore, no means has been available to routinely monitor this process factor prior to or at the beginning of the molding cycle, but rather within the thermoformer oven. It has also been the practice of the equipment operator to occasionally observe formed product exiting from the production mold either visually or by gauge measurement, and react to heat related product deficiencies via manual adjustments to one or more of the various oven heat zones, without knowing the actual sheet thickness as the sheet enters the forming mold.